Process to produce sanitary napkins with low tolerance between crimped side seal and absorbent core

ABSTRACT

A process to make a profiled disposable absorbent product is disclosed. The disposable absorbent product has a first sheet, a second sheet and a fibrous core. The core is disposed between the first and second sheets and both sheets are joined to each other along a continuous or discontinuous line. The process comprises the steps of joining the first sheet to the second sheet along a line following at least part of the periphery of the core with a distance to the core of 3 mm to 8 mm; and calendering the core between the sheets in a calender to approximately uniform density, the calendering being such that the core expands along its periphery and thereby reduces the distance of between the edge of the core and the line joining the first sheet to the second sheet by 1 mm or more. The process provides absorbent articles with very small distances between the core and the line of joining the topsheet and backsheet of the absorbent article.

FIELD OF THE INVENTION

The invention relates to a process for making a sanitary napkin havingan absorbent core between a topsheet and a backsheet and a crimped sideseal combining the topsheet and the backsheet around the core. Inparticular the process relates to making a product which has a smalldistance between the core and the crimped side seal of a sanitarynapkin.

BACKGROUND OF THE INVENTION

Historically the distance between the core edge and the side seal had tobe fairly large, typically up to 10 mm, in order to accommodatevariation, e.g,. caused by core growth during calendering of the coreprior to crimping. The process now devised uses a series of calanderingsteps to reduce this variation and hence allow the core edge and sideseal to be much closer. With a calandering step after forming the sideseal it becomes possible to make sanitary napkins having a small or evenno distance between the core edge and the crimped side seal.

Absorbent products are typically made out of three layers: a liquidimpervious backsheet; a liquid pervious topsheet; and an absorbent coresandwiched between the backsheet and the topsheet. When putting thesethree parts together during the manufacturing of absorbent products itis usual to have endless backsheet roll material and topsheet rollmaterial guided from above and below around a discontinuous absorbentcore structure.

The topsheet and backsheet are joined to each other at a certaindistance to the core in order to accommodate the variation of coredimensions due to the variation introduced when making the core. Thejoining can be continues or discontinues. The distance also has toaccommodate the alignment variations between core and the joining,equipment. The resulting product therefore necessarily has a certaindistance between the edge of the core and the innermost line of the edgeof the joining between the topsheet and the backsheet. In particular theusual crimp seal does not allow a tight tolerance between core edge andthe crimp seal, since foreign material between topsheet and backsheetwould cause the crimp seal to break or not even to form.

U.S. Pat. No. 4,592,708 discloses the problem of edge definition of thecore and tolerance between core edge and side seal edge. To overcomethis problem this disclosure suggests to calender the absorbent core intwo steps prior to combining the topsheet and the backsheet by crimping.This US patent therefore discloses an improvement step in the directionof the present invention however without satisfying the desire toapproach an almost unrecognisable distance between absorbent core edgeand joining edge.

Reference relating to the same subject are U.S. Pat. No. 4,888,231 andEP-A-429 393 both disclosing particular ways of calandering absorbentcores and joining the backsheet and topsheet around them. Howeverneither of these two disclosures provides the solution provided for bythe present invention.

Therefore one objective of the present invention is to provide a processby which absorbent articles, in particular sanitary napkins, can bemanufactured having a crimped side seal between the topsheet and thebacksheet and having sandwiched there between an absorbent corestructure while the distance between the side seal and the absorbentcore is minimised to a distance of not more than 6 mm with the intentionto eliminate this distance.

Another objective of the present invention is to provide the processwithout use of additional or new equipment relative to that which iscommercially available.

Yet another objective of the present invention is to provide thisprocess for absorbent structures having a profile, that is a varyingamount of absorbent material at different parts of the absorbentstructure particularly having more absorbent material in the centre ofthe absorbent structure. Ideally this is combined with calandering toproduce a uniform density of the absorbent structure despite theprofile. This is achieved by so called profiled calandering using aprofiled roll calender.

BRIEF DESCRIPTION OF THE INVENTION

The present invention relates to a precess to make profiled disposableabsorbent products having a first sheet, a second sheet and a fibrouscore. The core is disposed between the first and second sheet and bothsheets are joined to each other along a continues or discontinues line.The process comprises the steps of

joining the first sheet to the second sheet along a line following atleast partially the periphery of the core with a distance to the core of3 mm to 8 mm;

calandering the core between the sheets in a calender to approximatelyuniform density; the calandering being such that the core expands alongits periphery and thereby reduces the distance of step a), preferably by1 mm or more.

Most preferably the distance between core and the line of joining isreduced by 2 mm or more and best is a reduction to a distance of 0 mmafter the calendering step.

The joining should usually be along an endless line following all aroundthe periphery the absorbent core. The joining is preferably provided bya crimping process.

A separate calendering step prior to the joining can be included. If theabsorbent core is formed on a laydown drum with pockets, thenprecalendering on that drum as disclosed, for example, in U.S. Pat. No.4,592,708 or U.S. Pat. No. 4,859,388 can be included in order to furtherreduce the distance between core and line of joining.

DETAILED DESCRIPTION OF THE INVENTION

In the following the present invention will be described by reference tosanitary napkins as the absorbent article. However other absorbentarticles like baby diapers, adult incontinence products can also be madeby the process according to the present invention.

The design of sanitary napkins and the materials used in theirmanufacture are well known in the art. They are e.g. disclosed inEP-A-134 086, EP-A-130 848, EP-A-335 253, EP-A-336 578, WO 93/01783 andpending U.S. application Ser. No. 08/192240 of Feb. 4, 1994.

Typically sanitary napkins comprise an absorbent core made of fibrousmaterial which optionally further comprises absorbent hydrogelparticles. Such sanitary napkins cores are made by a laydown process asfor example disclosed in U.S. Pat. No. 4,592,708 or U.S. Pat. No.4,859,388. However other processes for preparing a fibrous absorbentcore may also be used according to the present invention. Preferablyabsorbent cores are profiled to have an increased amount of absorbentmaterial distributed towards the center relative to the periphery of thecores.

Once these absorbent cores are prepared they are placed consecutively ona conveyor belt in a spaced apart fashion. They are then sandwiched by atopsheet and a backsheet by introducing the respective roll materialfrom above and below receptively so as to form an endless band ofabsorbent products. The two sheets are then joined to each other aroundthe periphery of the absorbent core.

This joining can be provided by any of the typically processes usual inthe art of making absorbent products. For sanitary napkins crimping isoften used since it allows formation of the seal between topsheet andbacksheet following the periphery of the absorbent core relativelyclose. Crimping also does not require additional materials such asadhesives.

According to the present invention a calendering step is provided afterthe joining of the topsheet to the backsheet which is designed to expandthe absorbent core in order to approach a periphery coinciding with theline of joining between the topsheet and the backsheet.

To protect the crimp seal between the topsheet and the backsheet it isnecessary to use a profiled calendering, that is a calender roll whichfollows the material distribution of the absorbent core so as tominimise uneven expansion of the core periphery. The profiling of thecalender would typically follow the profile of the absorbent materialdistribution of the core. This will achieve an even expansion andprevent the splitting of the crimped seal from within. The profiledcalendering can be provided by a single profile roll calender or adouble calender system of a first calender of wide spacing and secondroll calender of narrower spacing with profiled roll.

Optionally, but not preferably, a calendering step of the absorbent coreprior to the formation of the seal between the topsheet and thebacksheet can be included. This calendering can, but does not have tobe, profiled.

Preferably the laydown drum calendering processes described in U.S. Pat.No. 4,592,708 and U.S. Pat. No. 4,859,388 can also be used together withthe present invention.

EXAMPLE

Below a comparison between a prior art process flow and the process flowaccording to the invention is described. A sanitary napkin having a coreof fibrous cellulose (also referred to as airfelt in the above citeddisclosures) and absorbent hydrogel particles can be produced by theseprocesses. The fibrous airfelt is carried by an air stream into whichthe absorbent hydrogel particles are mixed in order to create ahomogeneous mixture which is formed into an absorbent core in a laydownsection typically onto a laydown drum.

The absorbent cores are then calendered on the laydown drum inaccordance with U.S. Pat. No. 4,592,708 or U.S. Pat. No. 4,859,388 suchthat the resulting absorbent cores have a width of 56 mm and a thicknessof 30 mm. These cores are placed on a conveyor belt and spaced into adiscontinues sequence by use of a second conveyor belt of increasedspeed.

The cores carried by this conveyor belt to the first calender. Accordingto the prior art process the thickness is reduced to 10 mm which resultsin a product width of 54 mm due to the side expansion of the absorbentcores. According to the example of the present invention no calenderingis conducted at this stage.

Together with the topsheet and the backsheet, provided below and abovethe absorbent cores respectively, the absorbent product is combined inthe crimping section. Due to process variation the distance between theinner edges of the crimp seal on the longitudinal sides of the sanitarynapkin and the core has to be 5 mm on each side for the prior artprocess and 2 mm on each side for the process according to theinvention. 3 mm of the 5 mm are believed to be caused by the calenderprior to crimping.

This results in a distance between the inner edges of the crimp seal of58 mm+2×5 mm=68 mm for the prior art product. But only 56 mm+2×2 mm=60mm distance is required for the product made according to the presentinvention due to the smaller core width prior to the crimping and thereduced variation.

The process according to the present invention then has a secondcalendering step which expands the absorbent core to a total width of 58mm, leaving 1 mm on each side between the crimped seal and the absorbentcore while the absorbent core thickness is reduced to 10 mm as in theprior art.

The process according to the present invention allows the use of a webwidth for the topsheet and the backsheet of less than the prior artprocess. Depending on the particular absorbent product this results in areduced raw material consumption for all product width dependentcomponents of up to 20% in addition to the product improvements such asabsorbent core integrity and aesthetic impression.

    ______________________________________    Process Flow Example    Prior Art         Invention    ______________________________________    Lay down section to produce absorbent cores;    e.g. cores of 56 mm width and 30 mm height    First calender, profiled roll design    to densify the absorbent cores;    no calender    e.g. to a thickness of 10 m and a    width of 58 mm    Crimping section to join                      Crimping section to join    topsheet to backsheet                      topsheet to backsheet    around the absorbent core;                      around the absorbent core;    e.g. requiring 5 mm distance                      e.g. requiring 2 mm distance    to the core edge on each                      to the core edge on each    side, resulting in a crimp                      side, resulting in a crimp    seal distance of  seal distance of    58 mm + 2 × 5 mm = 68 mm                      56 mm + 2 × 2 mm = 60 mm    ______________________________________

What is claimed is:
 1. A process for making disposable absorbentproducts, said products having a first sheet, a second sheet and afibrous core, said core being disposed between said first and saidsecond sheet and said sheets being joined to each other along a line atleast partially following the periphery of said core with a reduceddistance to said core; said process comprising the steps ofa)joiningsaid first sheet to said second sheet along said line following at leastpartially the periphery of said core with a distance from said core tosaid line to be about 3 mm to about 8 mm prior to calendering of saidcore; and b)calendering said core between said sheets in a profiledcalender so that said core is expanded along its periphery and therebysaid distance is reduced such that the distance of the periphery of thecore to the line ranges from about 0 mm to about 3 mm.
 2. The processaccording to claim 1 wherein said distance of the periphery of the coreto the line ranges from about 0 mm to about 2 mm.
 3. The processaccording to claim 1 wherein said core is profiled having an increasedamout of absorbent material towards its center and said calender of saidstep b) is a profiled roll calender, which is adapted to said coreprofiling.
 4. The process according to claim 1 wherein said sheets arejoined along a line following everywhere the periphery of said core. 5.The process according to claim 1 wherein said joining of step a) isprovided by crimping.
 6. The process according to claim 1 wherein saidcore is formed on a profiled laydown drum and said process furthercomprises a step of pre-calendering said core on said drum.
 7. Theprocess according to any one of the preceding claims wherein saidabsorbent products are sanitary napkins.
 8. The process according toclaim 4 wherein said profiled roll calender is a double calendar system.9. The process according to claim 1 wherein the distance of theperiphery of the core to the line ranges from about 0 mm to about 1 mm.10. The process according to claim 1 wherein the distance of theperiphery of the core to the line is substantially nil.